Relax feerate requirements in TxBuilder::get_next_commitment_stats

We sometimes do not have easy access to the
`dust_exposure_limiting_feerate`, yet we are still interested in basic
stats on commitments like balances and transaction fees. So we relax
the requirement that the `dust_exposure_limiting_feerate` is always set
when `feerate_per_kw` is not 0.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -11979,21 +11979,12 @@ where
 	fn get_holder_counterparty_balances_floor_incl_fee(
 		&self, funding: &FundingScope,
 	) -> Result<(Amount, Amount), String> {
-		// We don't care about the exact value of `dust_exposure_limiting_feerate` here as
-		// we do not validate dust exposure below, but we want to avoid triggering a debug
-		// assert.
-		//
-		// TODO: clean this up here and elsewhere.
-		let dust_exposure_limiting_feerate =
-			if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
-				None
-			} else {
-				Some(self.context.feerate_per_kw)
-			};
 		let include_counterparty_unknown_htlcs = true;
 		// Make sure that that the funder of the channel can pay the transaction fees for an additional
 		// nondust HTLC on the channel.
 		let addl_nondust_htlc_count = 1;
+		// We are not interested in dust exposure
+		let dust_exposure_limiting_feerate = None;
 
 		let local_commitment_stats = self
 			.context

lightning/src/sign/tx_builder.rs

@@ -206,11 +206,8 @@ impl TxBuilder for SpecTxBuilder {
 		channel_type: &ChannelTypeFeatures,
 	) -> Result<NextCommitmentStats, ()> {
 		let excess_feerate_opt =
-			feerate_per_kw.checked_sub(dust_exposure_limiting_feerate.unwrap_or(0));
-		// Dust exposure is only decoupled from feerate for zero fee commitment channels.
-		let is_zero_fee_comm = channel_type.supports_anchor_zero_fee_commitments();
-		debug_assert_eq!(is_zero_fee_comm, dust_exposure_limiting_feerate.is_none());
-		if is_zero_fee_comm {
+			feerate_per_kw.checked_sub(dust_exposure_limiting_feerate.unwrap_or(feerate_per_kw));
+		if channel_type.supports_anchor_zero_fee_commitments() {
 			debug_assert_eq!(feerate_per_kw, 0);
 			debug_assert_eq!(excess_feerate_opt, Some(0));
 			debug_assert_eq!(addl_nondust_htlc_count, 0);